Which can help us pick up prime numbers in the range of a number you choose.

sieve_of_Eratosthenes.rb

puts "Enter an number greater than 1."
number = gets.chomp.to_i
puts "I'm gonna pick up prime numbers less than #{number}."

range = Array.new
initial_value = 2
(number - 1).times { range << initial_value; initial_value += 1 }

range.each do |i|
  i_square = i ** 2
  while i_square <= number
    range.delete(i_square)
    i_square += i
  end
end

puts range